The invention relates to an undercut anchor element with an anchor bolt, the elongate cylindrical shank of which is provided on its end which is to the rear in the fitting direction with a fixing head or a nut screwed onto a thread and has on its end which is to the front in the fitting direction a tightening head which widens in the shape of a truncated cone with a radially projecting annular projection provided on its free end, on which there is displaceably mounted a tubular base component which has multiple slots extending from its end which is at the front in the fitting direction and over a part of its length and forms a number of anchoring segments in the slotted region, the said base component having an internal diameter substantially equal to or somewhat greater than the external diameter of the cylindrical shank and having in its outer face a circumferential groove spaced from the end which is at the front in the fitting direction, wherein the said groove causes the end regions of the anchoring segments which are at the front in the fitting direction to be pivotable outwards whilst the material remaining in the base of the groove is deformed in such a way that in the state in which they are inserted into an appertaining fixing bore they are pivoted up by the introduction of a tensional force which displaces the anchor bolt and the base component relative to one another, whereby the anchoring segments taken together have the external shape of two truncated cone surfaces which abut one another with their end faces of smaller diameter and of which the base surfaces formed, on the one hand, on the end which is at the front in the fitting direction and, on the other hand, on the groove when they are not pivoted up each have a diameter which substantially corresponds to or is somewhat smaller than the external diameter of the base component, the truncated cone surface which is at the front in the fitting direction forming a cutting edge.
Such undercut anchor elements, which during fitting in a fixing bore in a substrate are anchored with positive engagement in the bore by penetration of a cutting edge in the wall of the fixing bore, have provoked increasing interest in recent years because they ensure a secure anchoring in the bore and thus fixing of components on a substrate without high radial forces acting on the wall of the bore, as is the case with the so-called spreading dowels which are anchored by frictional engagement in the bore. Because of the high radial spreading forces which are necessary for the anchoring by frictional engagement, the use of such spreading dowels is only possible in substrates made from concrete material or also solid stone or rock material which can be subjected to a correspondingly high load, and even with concrete substrates which can be loaded sufficiently it is only possible to provide fixing bores with considerable clearance from the boundary of the respective concrete component in order to ensure that the fixing bore does not break open when the spreading dowel is mounted. On the other hand, anchor elements to be fitted with positive engagement in bores are anchored on undercut surfaces which are larger than the diameter of the actual fixing bore and are produced either separately before the fitting of the anchor element by means of a special undercut drilling tool or—in the case of the undercut anchor elements under discussion here—are produced from the anchor element itself during fitting by penetration of chisel-like anchoring portions of an anchoring component of the anchor element itself penetrating radially into the wall of the bore. In this case radial forces are produced only in the zone of penetration of the chisel-like anchoring portions into the wall of the bore, i.e. are confined to a small region of the bore and are also relatively low in this region because the chisel-like construction of the anchoring portions allows them to penetrate into the wall of the bore even in the case of relatively low radial forces. In a known undercut anchor element of the aforementioned type (DE 105 20 130 A1) the base component identified there as an anchor sleeve has a relatively great longitudinal extent and the longitudinal slots forming the anchoring segments are guided a little further still beyond the groove and into the closed part of the anchor sleeve so that in the upper region extending beyond the circumferential groove they form individual arm portions which can be bent up under certain conditions, namely when during fitting the tightening head is drawn in by a predetermined additional amount into the anchoring segments. In this case the arm portions which are then expanded resiliently come to rest against the wall of the fixing bore and in turn exert radial spreading forces over a wide area there, but these are precisely the forces which should be avoided in the case of the undercut anchor elements under discussion here.